Thermal behaviour and non-isothermal crystallization kinetics of normal-alkanes and their waxy mixtures under quiescent conditions

Abstract

The thermal behaviour and the non-isothermal crystallization kinetics of ten pure odd- and even-numbered n-alkanes, namely n-C23H48, n-C25H52, n-C28H58, nand ten ternary prepared mixtures, and two commercial paraffin wax samples have been studied using differential scanning calorimetry (DSC). While the heating/cooling thermograms of n-C41H84, n-C44H90 and n-C50H102 exhibit only one major melting/crystallization transition, those of the shorter polymorphic n-alkanes display two main peaks corresponding to the liquid-solid and solid-solid transitions. Under an identical rate of temperature change, the shapes and peak-heights of the melting endotherms and cooling exotherms are found to differ noticeably. This distinctive feature is explained in terms of the established crystallographic descriptions of the crystalline rotator phases for these alkanes. Experimental results indicate that the peak-to-peak temperature difference depends on the carbon number but not on the cooling rate. The thermal traces for binary mixtures indicated the formation of either eutectic, solid solution or partially miscible systems. Their tendency to form solid solutions, however, increased with a larger number of components (i.e. n-alkanes) in the mixtures. At the same time, increased deviations from ideal behaviour were observed, especially at slow cooling rates. The DSC crystallization thermograms were transformed into relative crystallinity data and treated in terms of a simple kinetic model for non-isothermal crystallization of n-alkanes. The model is based on the fundamental equation of Ozawa for non-isothermal crystallization, the surface nucleation theory, and the growth rate theory for extended-chain crystals. Predictions from the proposed model are in excellent agreement with the experimental data for all n-alkanes at relatively low supercooling. Using a combination of thermodynamic and kinetic approaches, the developed model was extended successfully to describe the quiescent non-isothermal solidification process for paraffin mixtures at different cooling rates.